1. Field of the Invention
The present invention relates to a SAR radar system. The system permits detection of moving objects by means of a low-frequency radar with synthetic aperture (SAR). An important application is detection of moving objects concealed in forest vegetation from an airborne platform. In such a situation, both optical, infrared and microwave radiation are blocked to such an extent that it is not possible to carry out such detection.
2. Description of the Related Art
SAR is a known technique for two-dimensional high-resolution ground mapping. A platform, such as an aircraft or satellite, moves along a nominal straight path and illuminates a large ground area by means of an antenna. Short pulses, or alternatively long coded signals filtered by using pulse compression technique, are transmitted from the antenna and the return signal from the ground is received by the antenna and recorded along the straight path. By signal processing, high resolution is accomplished both along and transversely of the straight path. A condition for this is that the position of the antenna is known or can be calculated within a fraction of the wavelength and that the relative amplitude and phase of the transmitted and received radar signal are known. Moreover, the ground has to be invariable as the aircraft passes. The optimum geometric resolution that can be provided with SAR is determined by centre frequency and bandwidth of the transmitted signal and the aperture angle, over which the ground area is illuminated by the antenna, along the straight path.
The SAR technique has been applied in a very wide frequency range, about 20 MHz-100 GHz which corresponds to wavelengths of 3 mm-15 m. The choice of frequency determines largely which ground secures are to be reproduced since the backscattered return signal is affected above all by structures whose extent is of the wavelength size. Moreover, primarily the wavelength determines the capability of penetrating various ground layers, i.e. the penetration of the wave increases with a decreasing frequency. In connection with, for instance, vegetation, the attenuation is small for frequencies below 100 MHz and great for frequencies above 1 GHz. Thus the capability of penetrating vegetation decreases gradually with an increasing frequency, and a practical limit for detecting objects concealed in vegetation therefore is about 1 GHz. SAR systems which operate below and above 1 GHz, respectively, are in the following referred to as low-frequency and high-frequency systems, respectively.
Swedish Patent 8406007-8 (456,117) and the corresponding PCT Application SE85/00490 resulting in, inter alia, U.S. Pat. Nos. 4,866,446 and 4,965,582, and Swedish Patent Application 9503275-1 and the corresponding PCT Application SE96/01164, which are herewith incorporated by reference, disclose embodiments of low-frequency two-dimensional broadband SAR imaging.
Static objects in forest terrain can be detected with low-frequency SAR, i.e. with a wavelength in the range 0.3-15 m. The low frequencies have the property of penetrating the vegetation layer with little attenuation and only causing a weak back-scattering from the coarse structures of the trees. Thus, static objects, such as stationary vehicles, can be detected also in thick forest by combining low frequencies with SAR technique which gives resolution of wavelength size. This has been scientifically demonstrated in a plurality of experiments in recent years.
As described above, low-frequency SAR cannot detect objects that are moving. The high resolution of SAR arises by the imaging process using signals for a long time of integration. To enable sufficiently high resolution for detection, the radar must observe the object along a path which is of the same order as the distance to the object. This distance can be 20 km, i.e. for typical flying speeds the time of integration is about 100 s. During this time, an object must therefore be static within a fraction of the wavelength, i.e. the fraction of one meter. This fact makes is impossible in practice to detect moving camouflaged objects by using this technique. As a matter of fact, the speed of an object must be less than about 0.1 m/s for the object to be considered stationary.
It is a well-known fact that high-frequency SAR technique can be modified to detect and reproduce moving objects by using an array of narrow-lobe antennae. By arranging the antennae so that the antenna lobes are displaced in parallel it is possible by using signal processing to essentially eliminate all influence on the radar signal deriving from stationary objects. This GMTI function (ground moving target indication) can be implemented essentially in two different ways.
The first method is called DPCA (displaced phase centre antenna), which is used to eliminate stationary objects from the signals from two parallel-displaced antenna elements. This method utilises the fact that the signal, from all stationary objects, in the front and rear element, respectively, is repeated after a time interval in conformity with the platform moving the same distance as the element distance. After a delay, the signals from the stationary objects can thus be eliminated by subtraction. The drawback of this method is that it requires a calibrated and time-invariant radar system. A further problem with DPCA is that blind speeds arise, for which also moving objects are perceived to be stationary. The reason for this is that extinction also occurs when the phase change between the signals is a multiple of 2xcfx80. In practice this involves a demand for maximum antenna separation, which thus affects the detectable minimum radial speed.
The second method is called STAP (space-time-adaptive filtering) and is based on the covariance properties of the time signals for the different elements in the array antenna. The covariance matrix for stationary and moving objects, respectively, is different which is used by linearly combining signals in time and space so that a maximum ratio of desired to undesired signal is obtained. In practice, the co-variance matrix is estimated by taking random samples of the undesired signal, which together with a model of the desired signal forms an adaptive signal-adjusted space-time filter. The STAP technique is not restricted to the elimination of stationary objects as is the DPCA technique. Essentially all forms of undesired signal can be processed in the same manner provided that the covariance matrix can be estimated and that it differs from the desired signal. For example, also intentional or unintentional interference signals can be eliminated by the same method.
The basic questionxe2x80x94which the present invention intends to solvexe2x80x94is how to combine the technique of low-frequency SAR and the technique of detection of moving objects (GMTI) to produce signals which penetrate forest vegetation and at the same time permit detection of moving objects. The problem is especially the practical difficulty of providing on an airborne platform a sufficiently large radar antenna at low frequencies which has the same high directivity as a high-frequency narrow-lobe radar antenna so that the described methods for high-frequency SAR having the GMTI function can be used. The restricted physical space on board such a platform means essentially that low-frequency radar antennae are omnidirectional and have a low directly. The absence of directivity has two important consequences for a low-frequency SAR having the GMTI function, which mean that prior-art methods cannot be used.
First, the absence of directivity means a considerable problem of providing optimum performance for the GMTI function. The latter in fact requires that the directional sensitivity of the elements be as equal as possible, which is difficult to achieve if the directivity is low. The reason is that the antenna elements connect electromagnetically to the platform and thus the directional sensitivity changes. Consequently, the directional sensitivity is changed according to the exact position of the antenna element on the platform, the antenna elements having different direction properties which make the above-mentioned GMTI methods inefficient For this reason, the present invention introduces a solution in which the antenna elements are mounted in a translation-invariant and symmetric configuration in front of the platform.
Second, the absence of directivity means that a high signal sensitivity and geometric resolution require optimal coherent signal processing of radar data for a long time of integration. The signal integration corresponds to the object being illuminated over a large aperture angle. The aperture angle is de facto so large that the signal-processing methods that are used for moving objects in high-frequency SAR with the GMTI function are not applicable. For exactly the same reasons, the signal processing of stationary objects in low-frequency SAR is different from that used in high-frequency SAR.
The radar return at low frequencies can be described by a function f(t, r), where the time t parameterises the position of the radar antenna (obtained from the GPS and/or inertial navigation system of the platform) and r is the range from the radar antenna. It should be noted that bearing information is not available in radar data. The radar processing of stationary objects in low-frequency radar data transforms, with backprojection methods, f(t, r) to a two-dimensional SAR image g(x, p) in cylinder co-ordinates x and p1 where x is the azimuth distance along the path and p is the perpendicular distance away from the path. The position is unambiguous if the topography of the ground surface is known except for its mirror image through the flight path. The latter, however, can be distinguished by using the weak, but in this case still sufficient, directive efficiency of the antenna system. The position of the objects is thus obtained as the intersection between a circular cylinder (range cylinder) and two surfaces, one being a semiplane orthogonal with a cylinder axis (azimuth plane) and the other representing the ground surface, see FIG. 1. It is to be noted that a three dimensional reproduction is not possible without knowledge of the topography of the ground surface since there is no information about the third cylinder co-ordinate, the angle in the semiplane orthogonal with the straight path, in radar data. Such a reproduction can be provided by using a plurality of straight paths or a curvilinear flight path according to methods disclosed in Swedish Patent Application 9702331-1 and the corresponding PCT Application SE98/01147, which are herewith incorporated by reference.
In contrast to low-frequency SAR, high-frequency radar data are processed by means of methods which approximate the time variation in f(t, r) and are only applicable to narrow antenna lobes. The methods can be formulated in both the time and/or the Fourier planes. In these methods, especially movements of the platform which deviate from a straight path are corrected approximately by an angle-independent range correction. The latter approximation is applicable within a narrow angular sector and can therefore be applied in the case of a narrow antenna lobe. A similar method is applied also when the antenna lobe is controllable, so-called spotlight SAR. In this case, the range correction varies depending on the direction of the antenna lobe but is equal within the antenna lobe and, thus, approximate. Another common approximation in connection with high-frequency methods is power series development of the time or frequency variation of f(t, r). Typically, series developments are used including the square or cubic term. Also these approximations require a narrow angular sector.
For signal processing of low-frequency radar data, backprojection methods thus have several important advantages over high-frequency methods. On the one hand, general corrections of movements can be introduced into the algorithm, which are at the same time applicable to all angles and all image points. On the other hand, radar raw data are transformed to a SAR image without approximations. Finally, the demand for memory in calculation decreases drastically compared with the methods that are based on Fourier transformation.
Detection of stationary objects in forests using low-frequency SAR requires an optimal geometric resolution to discriminate the objects from the background. This results, of course, in a SAR with a great fractional bandwidth. Apart from achieving the optimal resolution, also the static fluctuations of the background which originate from the speckle effect are reduced. The speckle effect arises when the resolution is much greater than the wavelength (small fractional bandwidth) and contains a plurality of scatterers. The waves backscattered by the scattererers interfere with each other, and the resulting return signal thus is considerably dependent on the observation angle in relation to the resolution cell. Normally the resolution cell contains many independent scatterers, which results in a random amplitude and phase between different resolution cells, so-called speckle.
The detection of moving objects in forests by using low-frequency SAR is based on the fact that the speckle pattern is exactly reproducible if the same measuring geometry is repeated. By arranging the antenna elements in a translation-invariant configuration, all stationary objects, including the stationary background, will satisfy this requirement except for a time delay which corresponds to the mutual distance of the antenna elements divided by the speed of the platform. This means that the speckle pattern is uniform and can be eliminated using a GMTI function. Consequently it is not necessary to have a great fractional bandwidth for detection of moving objects but this can be accomplished by means of a considerably smaller bandwidth. The bandwidth requirement is instead determined by the desired degree of details for the measurements of the objects, i.e. for discrimination of different objects and measurement of their respective positions. It is to be noted that the measuring geometry for the antenna elements is repeated exactly only if the velocity vector of the platform is parallel with the connecting line of the elements, which in practice is not always the case. The method, however, is robust relative to such deviations since the speckle pattern changes only slowly, provided that the bandwidth is sufficient in principle, this means that the antenna elements need not necessarily be configured in a translation-invariant manner but that this is an advantage in order to provide optimum performance of the GMTI function.
For high-frequency SAR, the speckle pattern and, thus, the performance of the GMTI function are influenced by small motions of foliage or branches, for instance caused by the wind. Within a resolution cell the relative geometry and thus the interference between the backscattered waves change. This fact also involves problems of detecting moving objects at low speed since in this case also the background is perceived by the GMTI function as a moving object.
It is desirable to develop low-frequency SAR technique which enables detection of both moving and stationary objects. According to the discussion above, the bandwidth requirements, however, are not as strict for moving objects as for stationary. Since detection of moving objects can be carried out with a smaller bandwidth, the scanning capacity can increase since it is directly proportional to the geometric resolution. On the other hand, a complete measuring of moving objects requires a larger number of antenna elements and, thus, a larger number of signal channels, which reduces the scanning capacity. Thus the problem is the appearance of the signal pattern for a suitable adaptation of the requirements in respect of bandwidth and antenna channels between the modes, see also under item 5 at the end of the specification.
In view of that stated above, it would be great progress if the SAR technique could be improved to allow moving objects to be detected and reproduced by means of a low-frequency SAR radar. In this way, stationary as well as moving objects can be discovered and measured. The object of the invention is to solve this problem, which is achieved by giving the invention the features that appear from the accompanying independent claim. Suitable embodiments of the invention are stated in the remaining claims.